Comparative evaluation of shear bond strength of TheraBase and composite resins to dentin: an in vitro study

INTRODUCTION

Dental caries, a chronic disorder, is defined as a disease that causes a shift in the bio-film towards acid-producing microbes, eventually culminating in mineral loss from hard dental tissue [1]. The standard method of treating deep cavities involves completely removing all carious dentin. Due to technology improvements, researchers are focusing on vital pulp treatments (VPTs) as feasible alternatives to root canal treatments. The application of bio-active materials resulted in improved understanding of pulp regeneration and vascularization. Indirect pulp capping (IPC) is the process of applying a capping material over affected dentin, covering unexposed pulp. Recent studies have shown that sealing cavitated lesions with bonded composite restorations can halt caries progression for at least a decade, implying that complete caries removal is not al-ways required, thus leaving caries-affected dentin [1]. The total success rate of IPC in one of the studies was 94% at the end of 12 months. This outcome was consistent with current studies showing excellent success rates for IPC as a therapeutic option for deep carious lesions, ranging from 92% to 97% [2]. The longevity of enamel bonding is enhanced by micro-mechanical retention; however, bonding to dentin is comparatively challenging, mainly because of the hydrophilicity, high organic content, tubule-dominated micro-structure of dentin, presence of a smear layer, and intrinsic surface wetness [3]. Chemical adhesion is through ionic interactions between polycarboxylate radicals and hydroxyapatite, and has a beneficial effect against hydro-lytic degradation. However, caries-affected dentin has higher precipitates and b-tricalcium phosphate minerals (whitlockite) in dentinal tubules, which are less soluble than hydroxyapatite and may impair ionic interaction. Potentially unstable adhesive surfaces can degrade slowly and steadily due to water absorption. In such cases, dentin bio-modification is critical for improving bonding stability [4]. Therefore, ion-releasing base/liner or restorative materials are expected to exhibit acceptable qualities at the interface of tooth and material [3, 5]. Clinical success and durability of restorations depend on the restorative material’s capacity to achieve good adhesion to the surface of dentin, along with resistance to dislodging forces in the oral cavity [6]. These dis-lodging forces are measured in terms of compressive strength, tensile strength, and shear bond strength. Shear bond strength is the resistance to forces displacing restorative material from the tooth structure. Conventional cements, which are most commonly used as base under restorative materials, show some inherent disadvantages, such as solubility, pulpal irritation, incompatibility with composite resins, etc. As an alternative, flowable composite resin can be used as a liner. To overcome these drawbacks, newer materials are being introduced. TheraBase (BISCO, USA) is a dual-cure self-adhesive base/liner, with an exceptional property of both calcium and fluoride ion release. TheraBase chemically bonds to tooth structure, is self-adhesive, and does not require etching and bonding to the tooth structure [7]. Moreover, it contains 10-methacryloxydecyl dihydrogen phosphate (10-MDP) that promotes adhesion, ensuring an optimal and reliable bonding to the dentin [8]. Recent adhesive systems attempted to streamline the clinical procedure, but this simplification of procedure can reduce bond strength due to different structural characteristics of the enamel and dentin [9]. They involve two protocols: total-etch bonding technique and self-etching system that uses a simpler approach, and is distinguished by its components and bonding protocol [10].

OBJECTIVES

The aim of this study was to evaluate the shear bond strength of TheraBase to dentin, and compare it with composite resin-bonded material using etch-and-rinse and self-etch adhesive systems. The null hypothesis was that there is no difference between the shear bond strength of TheraBase, etch-and-rinse, and self-etch bonding systems.

MATERIAL AND METHODS

Forty-five intact non-carious premolars extracted for orthodontic purposes were selected as per inclusion criteria for the study. The teeth selected were thoroughly debrided and stored in a saline solution.

SPECIMEN PREPARATION

Firstly, each tooth was embedded into a rectangular block (1 cm × 4 cm) made from self-cure acrylic resin. The buccal surface of each tooth was prepared by exposing superficial dentin using a tapered fissure diamond point to a depth of 2 mm. The prepared samples were randomly allocated into three groups.
Group 1: Cylindrical silicon moulds of 4 mm height and 1.8 mm width were made. TheraBase was directly applied with a mixing tip to the exposed buccal surface of dentin using these silicon moulds, and cured for 20 sec with light cure unit (3M, Elipar, USA). No etching or bonding was done in this group.
Group 2: Etching was done with 37% phosphoric acid (Prime Dental, Mumbai, India) for 15 sec, followed by rinsing for 5 sec. Two coats of etch-and-rinse bonding agent (5th generation; Single Bond 2, 3M ESPE, USA) were applied for 15 sec with gentle agitation using an applicator tip. Gentle air thinning was done for 5 sec, followed by light curing (3M, Elipar, USA) for 20 sec. A cylindrical projection of composite (Filtek Z350 XT, 3M, USA) was made perpendicular to the prepared buccal surface of teeth using silicon moulds. The composite was placed in two increments of 2 mm, with each increment cured according to the manufacturer’s instructions.
Group 3: One coat of self-etch bonding agent (7^th generation; Single Bond Universal Adhesive) was applied, followed by air drying for 10 sec. Second coat was applied with agitation using a applicator tip for 30 sec, and gently air-thinned for 5 sec and light-cured (3M, Elipar, USA) for 20 sec. Cylindrical projections of composite were made perpendicular to the prepared buccal surface of teeth using silicon moulds. The composite was placed in two increments of 2 mm, with each increment cured according to the manufacturer’s instructions.

TESTING OF SHEAR BOND STRENGTH

Testing of samples for shear bond strength was performed using an universal testing machine, with a crosshead speed of 0.5 mm/min. A chisel-shaped blade was employed for force application, oriented parallel to the dentin, at the interface of dental adhesive or Thera­Base and dentin. Force, at which failure occurred, was noted in Newtons and the value was divided by the surface area to obtain shear bond strength value in MPa.

STATISTICAL ANALYSIS

Data were obtained and tabulated using Microsoft Excel version 13.0, followed by statistical analysis with IBM SPSS version 21.0. Data were found to be continuous, and hence mean and SD for max. load and shear bond strength were obtained. To compare between groups, independent t-test was applied. All statistical tests were conducted with confidence interval at 95% and p < 0.05 considered statistically significant.

RESULTS

The mean distribution of max. load in group 1, 2, and 3 showed that the mean maximum load was 67.34 ± 6.65, 148.70 ± 20.12, and 172.15 ± 7.64, respectively. The mean distribution of shear bond strength in group 1, 2, and 3 was 9.52 ± 0.93, 21.02 ± 2.84, and 24.34 ± 1.08, respectively (Table 1). Comparison of the shear bond strength in group 1 and group 2 revealed that in group 2, the shear bond strength was significantly higher than in group 1, with the difference of –11.50 (p < 0.05) (Table 2).
When comparing the shear bond strength between groups 1 and 3, it was found that in group 3, the shear bond strength was significantly higher than in group 1, with the difference of –14.82 (p < 0.05) (Table 2). When comparing the shear bond strength between groups 2 and 3, it was found that in group 3, the shear bond strength was higher than in group 2, but the difference was not as statistically significant (p < 0.05) (Ta-ble 2).

DISCUSSION

Modern restorative dentistry techniques incorporate adhesive bonding to the tooth structure, enhancing the bio-mechanical and esthetic quality of restorations. The bond’s interface continues to be the biggest obstacle when applying an adhesive restoration or base, despite significant advancements in adhesive technology [11].
The restoration’s susceptibility to secondary caries is prevented by adding agents, which help in elimination of the remaining bacteria as well as promote re-minera­lization of the affected dentin and enamel [12]. The results demonstrated that even though TheraBase had a mean shear bond strength, which was lower than that of etch-and-rinse and self-etch bonding systems, its strength as a base under a restorative material is quite good. Various studies have reported the mean shear bond strength of glass ionomer cement (GIC) to be within a range of 4.0-7.6 Mpa [13, 14]. Therefore, when assessing conventional cements, such as GIC, ap-plied as a base under restorative materials, the bond strength of TheraBase is comparable. TheraBase utilizes Thera technology, in which a hydrophilic matrix is developed to allow for exchange of ions. The reaction starts with water incorporated into the matrix, resulting in the release of calcium hydroxide and fluoride ions. Dual-curing polymerization helps in polymerization of the material in deep cavities, where light does not reach [7].
TheraBase has an adhesion promoting monomer MDP that is a functional monomer with a hydrophilic nature. MDP enhances the micro-mechanical bonding, as it has the quality to react with the tooth substrate minerals chemically [14]. In addition to having the strongest interactions with hydroxyapatite, 10-MDP have the most hydrolytically stable bonds with calcium [8]. The probable reason for TheraBase having the least shear bond strength when compared with other groups, might be the initial availability of calcium ions needed for chemical adhesion in the formation of inter-facial hydroxyapatite layer. TheraBase has a gradual increase in calcium ion release over time; hence, the initial availability might be lesser than expected. In addition to TheraBase’s good bond strength, it has calcium and fluoride release along with hydroxyl ions as free radicals to prevent the recurrence of secondary caries. The hydroxyl ions offer an alkaline pH (about, 12), aiding in mineralization through enzymatic inhibition and production of alkaline phosphatase [15]. The release of fluoride results in a higher resilience of enamel to acid breakdown by producing fluorapatite (Ca10(PO4)6F2). When fluoride is administered to the teeth, a layer of calcium fluoride (CaF2) precipitates on the enamel surface. This acts as a mineral reservoir area and can function as a physical barrier to prevent acid interactions with the enamel [16]. Along with ion release, TheraBase also has a high flexural strength that makes it stronger and more fracture-resistant. As a base in the cavity, it acts as a shock absorber, bearing the stress from occlusal forces without fracturing. TheraBase is claimed to be stronger when compared with other base materials, such as glass ionomer cement and resin-modified glass ionomers [7]. Additionally, its radiopaque nature helps easy identification on radiographs. It generates an alkaline pH of 9 [7], thereby promoting an unfavorable environment for survival of the existing bacteria and preventing further bacterial invasion. Dentin bonding is based on the mechanism of resin-dentin interface formation at the inter-diffusion zone, called hybrid layer. Despite the fact that the hybrid layer developed by total etch systems is thicker than that made by self-etch systems, a comparison of bond strength between the two has produced controversial results. The thickness of hybrid layer and dentin bond strength are most likely proportional to resin tags interlocking with collagen fibers. The resin tags produced by etch-and-rinse adhesive systems are significantly greater in length than those produced by self-etching adhesive ones, but both techniques provide a continuous and homogenous hybrid layer [17]. Single bond universal bonding agent result-ed in having the mean highest bonding strength of 24.3 Mpa. This may be explained by the mechanism of bonding of single bond universal (SBU), in which simultaneous de-mineralization and infiltration of adhesive are seen in the tooth substrates. Furthermore, it contains 10-MDP, enhancing micro-mechanical adhesion. Its chemical interaction increases the hydrolytic stability of hybrid layer, resulting in long-term bonding [14]. SBU contains hydroxyethyl methacrylate (HEMA) that enhances dentin wettability, resulting in improved exposure of hydroxyapatite crystals to the bonding agent, thus lowering their dissolution [18]. SBU has a high amount of Vitrebond copolymer that chemically interacts with apatite crystals, and disperses stresses at the adhesive contact. It is also known to provide stability against deterioration due to humidity. Additionally, nano-fillers in SBU create a thicker layer of adhesive that acts as a shock absorber and relieves inter-facial strains between the stiff dentinal interface and resin composite that goes through polymerization shrinkage [19]. As self-etch adhesives are less acidic, they de-mineralize dentin more superficially than total-etch bonding agents [20]. Adper Single Bond 2, as an etch-and-rinse system showed a weaker bond strength when compared with self-etch systems. This could be due to the highly aggressive acids, which can expose collagen and reduce bond strength by not allowing the penetration of adhesive resins entirely, resulting in a collagenous layer of dentin that is weak, unfiltered, and prone to deterioration over time [21].
Further studies are required for the evaluation of the performance of TheraBase in vitro and in vivo, and to investigate the stability of bond over time. Thera­Base has been used under composite restorations successfully [7], but further research are needed to evaluate the bond strength of composite and other restorative materials.

CONCLUSIONS

Traditionally, pulp protection is done using cement-based materials. TheraBase can be employed as a good alternative to glass ionomer cement in deep compo-site resin restorations due to its comparable bond strength. Its ion-releasing properties make it a promising prospect towards the future of bio-active materials.

DISCLOSURES

1. Institutional review board statement: Not applicable.
2. Assistance with the article: None.
3. Financial support and sponsorship: None.
4. Conflicts of interest: The authors declare no potential conflicts of interest concerning the research, authorship, and/or publication of this article.

References